1. Field of the Invention
This invention relates generally to a nonvolatile memory devices. More particularly this invention relates to circuits and methods for executing protocols for communicating between nonvolatile memory arrays and external systems. Even more particularly, this invention relates to circuits and methods for controlling operation of multiple NAND and NOR flash memory arrays and communicating between the NAND and NOR flash memory arrays and external control systems.
2. Description of Related Art
Nonvolatile memory is well known in the art. The different types of nonvolatile memory include Read-Only-Memory (ROM), Electrically Programmable Read Only Memory (EPROM), Electrically Erasable Programmable Read Only Memory (EEPROM), NOR Flash Memory, and NAND Flash Memory. In current applications such as personal digital assistants, cellular telephones, notebook and laptop computers, voice recorders, global positioning systems, etc., the Flash Memory has become one of the more popular types of Nonvolatile Memory. Flash Memory has the combined advantages of the high density, small silicon area, low cost and can be repeatedly programmed and erased with a single low-voltage power supply voltage source.
A present day flash nonvolatile memory is divided into two major product categories such as the fast random-access, asynchronous NOR flash nonvolatile memory and the slower serial-access, synchronous NAND flash nonvolatile memory. NOR flash nonvolatile memory as presently designed is the high pin-count memory with multiple external address and data pins along with appropriate control signal pins. One disadvantage of NOR flash nonvolatile memory is as the density is doubled, the number of its required external pin count increases by one due to the adding of one more external address pin to double the address space. In contrast, NAND flash nonvolatile memory has an advantage of having a smaller pin-count than NOR with no address input pins. As density increases, the NAND flash nonvolatile memory pin count is always kept constant. Both main-streamed NAND and NOR flash nonvolatile memory cell structures in production at the present time use one charge retaining (charge storage or charge trapping) transistor memory cell that stores one bit of data as charge or as it commonly referred to as a single-level program cell (SLC). They are respectively referred as one-bit/one transistor NAND cell or NOR cell, storing a single-level programmed data in the cell.
The NAND and NOR flash nonvolatile memories provide the advantage of in-system program and erase capabilities and have a specification for providing at least 100K endurance cycles. In addition, both single-chip NAND and NOR flash nonvolatile memory products can provide giga-byte density because their highly-scalable cell sizes. For instance, presently a one-bit/one transistor NAND cell size is kept at ˜4λ2 (λ being a minimum feature size in a semiconductor process), while NOR cell size is ˜10λ2. Furthermore, in addition to storing data as a single-level program cell having two voltage thresholds (Vt0 and Vt1), both one transistor NAND and NOR flash nonvolatile memory cells are capable of storing at least two bits per cell or two bits/one transistor with four multi-level threshold voltages (Vt0, Vt1, Vt2 and Vt03) in one physical cell. The multi-level threshold voltage programming of the one transistor NAND and NOR flash nonvolatile memory cells is referred to as multiple level programmed cells (MLC).
Currently, the highest-density of a single-chip double polycrystalline silicon gate NAND flash nonvolatile memory chip is 64 Gb. In contrast, a double polycrystalline silicon gate NOR flash nonvolatile memory chip has a density of 2 Gb. The big gap between NAND and NOR flash nonvolatile memory density is a result of the superior scalability of NAND flash nonvolatile memory cell over a NOR flash nonvolatile memory. A NOR flash nonvolatile memory cell requires 5.0V drain-to-source (Vds) to maintain a high-current Channel-Hot-Electron (CHE) injection programming process. Alternately, a NAND flash nonvolatile memory cell requires 0.0V between the drain to source for a low-current Fowler-Nordheim channel tunneling program process. The above results in the one-bit/one transistor NAND flash nonvolatile memory cell size being only one half that of a one-bit/one transistor NOR flash nonvolatile memory cell. This permits a NAND flash nonvolatile memory device to be used in applications that require huge data storage. A NOR flash nonvolatile memory device is extensively used as a program-code storage memory which requires less data storage and requires fast and asynchronous random access.
The current consumer portable application requires a high speed, high density, and low cost NVM memory solution. The Serial Peripheral Interface has been widely used in serial flash nonvolatile memory devices. The Serial Peripheral Interface Bus or SPI bus is a synchronous serial data link protocol from Freescale Semiconductor Inc., Austin, Tex. 78735 (formally Motorola Inc.). The SPI bus operates in full duplex mode where devices communicate in master/slave mode and the master device initiates the data frame. A single Master device and multiple slave devices are allowed with individual slave select (chip select) lines. The SPI bus specifies four logic signals—SCLK—Serial Clock (output from master); MOSI/SIMO—Master Output; Slave Input (output from master); MISO/SOMI—Master Input, Slave Output (output from slave); and SS—Slave Select (active low; output from master).
The SPI bus has some of the following disadvantages: 1. SPI has no in-band addressing (multiple slave devices on a shared bus must have separate select lines or out-of-band chip select signals to address separate slaves shared buses). 2. SPI supports only one master device. 3. Without a formal standard, validating conformance is not possible.
The Serial Quad I/O™ (SQI™) is a 4-bit multiplexed I/O serial interface from Silicon Storage Technology, Inc., Sunnyvale, Calif. 94086. The SQI Interface provides Nibble-wide (4-bit) multiplexed I/O's with an SPI-like serial command structure and operation. The SQI bus consists of a Serial Clock (SCK) to provide the timing of the serial interface. Commands, addresses, or input data are latched on the rising edge of the clock input, while output data is shifted out on the falling edge of the clock input. The Serial Data Input/Output (SIO[3:0]) transfers commands, addresses, or data serially into a device or data out of a device. Inputs are latched on the rising edge of the serial clock. Data is shifted out on the falling edge of the serial clock. Chip Enable CE# provides enables a device by a high to low transition. The Chip Enable must remain low for the duration of any command sequence; or in the case of Write operations, for the command/data input sequence. Rather than the full-duplexed operation with the MOSI/SIMO—Master Output; Slave Input (output from master) and MISO/SOMI—Master Input, Slave Output (output from slave) of the SPI interface, the SQI functions as a half-duplex with the command, address, and data signals being transferred from the master to the slave and the Serial Data Input/Output bus reversing direction to have data and status being transferred from the slave to the master. With an 80 Mhz system clock rate, the maximum sustained data transfer rate is 320 Mbit/sec. The demand for future applications is for a maximum sustained data transfer rate of more than 1 Gbit/sec.